This invention relates to a method of an apparatus for measuring the flow of red blood cells flowing in a microvascular bed. The present invention relates, more particularly, to a method of an apparatus for measuring the flow of red blood cells flowing in a microvascular bed, using the Doppler scattering of coherent light. The invention can be practiced in connection with the measuring of the blood flow parameters of kidney tissues, brain tissues, liver tissues, tissues of other organs, and local cutaneous tissues, as well.
The study of pharmacologic agents and pathophysiologic states requires a technique of measuring the tissue blood flow in internal organs, in the microvascular bed of the skin and the like, its distribution in different regions of the tissues, and its variation with time. This is especially true in the kidney, where interarenal redistribution of flow is one of the major effects of drugs and hemodynamic changes.
Known techniques of measuring regional renal blood flow include radioactive indicator washout, implanted hydrogen electrode indicator dilution technique, autoradiography, angiography, implantation of .beta.-ray detectors and radioactive microsphere trapping. Each of these known techniques has serious drawbacks for the monitoring of tissue perfusion during physiologic experiments. The radioactive tracer washout can be used dynamically, but there is doubt as to the localization of the abstract compartments which it defines, and whether this localization is the same in all physiologic states. The hydrogen electrode method is invasive, and may cause alterations in local flow. The same applies to implantable radiation detectors. The radioactive microsphere method is precise and localized, but it is destructive, and only a small number of data points may be taken in a single subject. It cannot be used to study dynamic changes in real time. The same is true of autoradiography. Angiography is not quantitative and requires the injection of contrast media which may disturb renal function.
It has been proposed in U.S. Pat. No. 3,511,227 to C. C. Johnson entitled "Measurement of Blood Flow Using Coherent Light" issued May 12, 1970 that the rate of blood flow within a blood vessel can be determined by measuring the Doppler frequency shift of coherent radiation, which can be produced by a laser, by directing a coherent light beam into the blood stream of a patient, and comparing the frequency of the scattered light radiation with the frequency of the original beam, the difference being a measure of the blood flow rate. This technique may be suitable for measuring the flow rate within a relatively large vessel, using an optical catheter or needle, or in some cases, by selecting a given wavelength which will penetrate the vessel with a venipuncture. On the other hand, the technique cannot be used accurately to measure the velocity of motion of red blood cells in a microvascular bed of an organ, for example, to measure the local renal cortical blood flow parameter or the local cutaneous blood flow parameter.
It has been reported, on the basis of preliminary experiments, that if the coherent monochromatic light of a laser is used to illuminate tissues, the light scattered from the tissue has a broadened spectrum. The broadening is believed to be a result of the Doppler frequency shift sustained by light when it is scattered from red cells moving in the microvessels. See Stern, "In vivo evaluation of microcirculation by coherent light scattering", Nature, Vol. 254, pages 56-58, March 1975.